The present invention relates to an electromagnetic flowmeter and, more particularly, to an electromagnetic flowmeter having a function of suppressing fluctuations in measurement flow rate due to commercial power noise mixed in a fluid.
When a fluid to be measured by an electromagnetic flowmeter passes through a pump or electromagnetic valve, 50-Hz/60-Hz commercial power noise caused by such a device may be mixed in the fluid. If such commercial power noise is mixed in the fluid, since the commercial power noise is superimposed on the signal electromotive force obtained from the detector, differential noise having a differential frequency between the commercial power frequency and a sampling frequency is also superimposed on the DC flow rate signal obtained by sampling the signal electromotive force. When such a DC flow rate signal is loaded as a digital flow rate signal, fluctuations occur in a measurement flow rate due to the influences of the differential noise.
Conventionally, as an electromagnetic flowmeter designed to suppress fluctuations in measurement flow rate which are caused by such commercial power noise, an electromagnetic flowmeter having a band elimination filter (to be referred to as a BEF hereinafter) 120 for eliminating differential noise in a DC flow rate signal 112A as shown in FIG. 12 has been proposed (e.g., Japanese Patent Laid-Open No. 2000-258211).
Referring to FIG. 12, a detector 100 applies a magnetic field to a fluid in a pipe line on the basis of a predetermined AC exciting current, and detects/outputs the signal electromotive force generated in a fluid as a detection signal. A converter 110 outputs a predetermined AC exciting current to the detector 100, and calculates/outputs a flow rate in the pipe line by performing signal processing for the detection signal from the detector 100.
As shown in FIGS. 13A to 13E, an exciting section 116 outputs an AC exciting current which has a predetermined frequency and is formed from a rectangular wave on the basis of an exciting signal 117C from a switching section 117.
A coil 100c of the detector 100 is excited by the AC exciting current from the converter 110 to apply a predetermined magnetic field to a fluid flowing in a pipe line 101. This generates a signal electromotive force having an amplitude corresponding to the flow velocity of the fluid.
This signal electromotive force is detected by electrodes 100a and 100b disposed on the inner wall of the pipe line 101 at positions to oppose each other, and is output as a detection signal to the converter 110.
In the converter 110, a first-stage amplification section 111 attenuates the low-frequency components of the detection signal obtained from the detector 100 by using a high-pass filter or the like so as to attenuate pulse-like noise and low-frequency noise mixed in this detection signal, AC-amplifies the signal, and outputs the resultant signal as an AC flow rate signal 111A.
A sample/hold section 112 samples each waveform trailing edge portion (hatched portion) of the AC flow rate signal 111A from the first-stage amplification section 111, which portion is affected little by the magnetic flux differential noise produced by the exciting coil 100c, and outputs the resultant signal as the DC flow rate signal 112A. The band elimination filter 120 attenuates a differential noise component with a frequencyΔf=|fn−fex|which corresponds to the difference between an exciting frequency fex and a commercial power frequency fn contained in the DC flow rate signal 112A from the sample/hold section 112.
An arithmetic processing section 114 loads the DC flow rate signal 112A output from the sample/hold section 112 through the BEF 120 as a digital flow rate signal, and calculates a measurement flow rate by executing predetermined arithmetic processing. An output section 115 then converts the flow rate into a predetermined flow rate signal (loop current) and outputs it.
In this manner, a measurement flow rate in which fluctuations caused by differential noise is suppressed is obtained.
An electromagnetic flowmeter is also available, which obtains an output voltage ES indicating the difference between electrode voltages EA and EB obtained from the respective detection electrodes 100a and 100b so as to reduce commercial power noise that causes differential noise instead of directly reducing the differential noise in the above manner, as shown in FIG. 14.
In general, commercial power noise mixed in a fluid tends to be equally mixed as common mode noise Nc in the respective detection electrodes 100a and 100b. When such common mode noise Nc is mixed in the fluid, the electrode voltages EA and EB generated between the respective detection electrodes 100a and 100b and ground potential 100d are given byEA=SA+NCEB=SB+NCwhere SA and SB are the signal electromotive forces generated by the detection electrodes 100a and 100b. 
At this time, since the signal electromotive forces are expressed bySA=−SBwhen the difference between these electrode voltages EA and EB is calculated by a subtracter 151, the output voltage ES with the common mode noise Nc being canceled is obtained:ES=EA−EB=2SA
In contrast to this, when the electrode voltages EA and EB are added by an adder 152, flow rate signals cancel each other, a noise voltage EN representing commercial power noise is obtained:EN=EA+EB=2NC
By extracting a commercial power frequency from this noise voltage and performing excitation in synchronism with the extracted frequency, the arithmetic processing section 114 can perform arithmetic operation in the subtracter 151.
More specifically, like the sample/hold section 112 shown in FIG. 12, the arithmetic processing section 114 samples the electrode voltages EA and EB in half cycles to obtain ES by using EA and EB which are phase-shifted half cycle. In this case, since the excitation timing is synchronous with the commercial power frequency, the common mode noise Nc is equally mixed in EA and EB which are phase-shifted half cycle. This eliminates the phase shift and can effectively cancel out the common mode noise Nc without using the subtracter 151.
In this manner, a measurement flow rate in which fluctuations caused by differential noise are suppressed is obtained.
Such a conventional electromagnetic flowmeter, however, additionally requires an analog signal processing circuit for suppressing fluctuations caused by differential noise having the differential frequency between a commercial power frequency and a sampling frequency. This leads to an increase in manufacturing cost and an increase in power consumption. In a two-wire electromagnetic flowmeter in which the maximum current consumption is limited to 4 mA or less, in particular, an increase in power consumption poses a serious problem.
As in the former case, when differential noise is to be eliminated by a BEF, a relatively narrow frequency band near the differential frequency must be effectively eliminated. In this case, a filter circuit with a certain scale is required, and an increase in power consumption is inevitable.
As in the latter case, when commercial power noise as a noise source is to be attenuated, the electromagnetic flowmeter requires a subtraction circuit for accurately eliminating commercial power noise from electrode voltage in opposite phases, and an addition circuit for accurately extracting a commercial power frequency from commercial power noise, resulting in an increase in current consumption.